Semiconductor Device

ABSTRACT

In an n-channel type power MISFET, a source electrode in contact with an n + -semiconductor region (source region) and a p + -semiconductor region (back gate contact region) is constituted with an Al film and an underlying barrier film comprised of MoSi 2 , use of the material having higher barrier height relation to n-Si for the barrier film increasing the contact resistance to n-Si and backwardly biasing the emitter and base of a parasitic bipolar transistor making it less tending to turn-on, thereby decreasing the leak current of power MISFET.

BACKGROUND OF THE INVENTION

The present invention concerns a semiconductor device and, more inparticular, it relates to a technique which is effective to be appliedto a semiconductor device having a power MISFET (Metal InsulatorSemiconductor Field Effect Transistor).

Transistors that can be used for high power application of several wattsor higher are referred to as power transistors, for which variousstructures have been studied.

Among them, power MISFETs includes those referred to as a vertical typeand lateral type and they are classified structure, for example, into atrench type and planer type in accordance with the structure of a gateportion.

The MISFET described above has a structure in which plural MISFETs eachof a fine pattern are connected (for example, by the number of severalthousands) in parallel in order to cope with high power.

Barrier films (barrier metal films) are formed to connection portionswith wirings or semiconductor substrates for preventing formation ofundesired reaction layers or deposition of underlayer material caused bythe contact of material constituting them.

For example, Japanese Patent Laid-Open No. 2001-127072 discloses atechnique of forming titanium tungsten between source wirings (15) and asemiconductor substrate (1S) of a power MISFET.

SUMMARY OF THE INVENTION

The present inventors have been now under research and development forsemiconductor devices, particularly, power MISFETs.

In such a power MISFET, it has been studied for refinement of elements,for example, reduction of the width of a trench to which a gate portionis formed since it us necessary to decrease on-resistance (Ron), gatecapacitance (Qg) and gate drain capacitance (Qgd).

That is, for decreasing the on-resistance, it is necessary to increasethe channel width per unit area and the channel width per unit area canbe increased by decreasing the width of the trench to which the gateportion is formed. Further, when the width of the trench to which thegate portion is formed is decreased, an opposing area between the gateportion and the drain portion at the rear face of a substrate can bemade smaller to decrease the capacitance (Qgd).

On the other hand, while power MISFETs have various application uses,severe standards for IDSS (leak current between source and drain when avoltage is applied to the drain at a 0 V state between gate and source)have been imposed to many devices used for measuring instruments.Examples of devices used for measuring instruments include thoseproducts in which light emitting devices (LED), photoelectronic devices(solar cells) and power MISFET are formed into one package.

In power MISFET used for the products described above, it is anessential condition that the leak current (IDSS) is low, for example,with the reason that it gives undesired effects on the measured value.

However, when the present inventors have made a study by decreasing thewidth of the trench to which the gate portion is formed for therefinement of the element, for example, by reducing the trench widthfrom 0.8 μm to 0.5 μm, it has been observed a phenomenon that IDSSincreased by about one digit.

The present invention intends to decrease a leak current of asemiconductor device, particularly, a power MISFET.

The invention also intends to improve the characteristics of asemiconductor device, particularly, a semiconductor device having powerMISFET.

The foregoing and other objects and the novel features of the inventionwill become apparent by referring to the descriptions of the presentspecification and appended drawings.

Among the inventions disclosed in the present application, the outlineof typical inventions will be described briefly below.

The semiconductor device of the invention comprises

(a) an MISFET formed in a semiconductor substrate, including:

(a1) a gate portion comprising a first conductor formed on the surfaceof the semiconductor substrate;

(a2) a source portion comprising semiconductor region of a firstconduction type on the surface of the semiconductor substrate and in thesemiconductor substrate at the end of the gate portion;

(a3) a drain portion comprising a semiconductor region of the firstconduction type; and

(a4) a channel portion arranged between the source portion and the drainportion, and

(b) a conductive portion formed above the semiconductor substrate and incontact with the source portion and the channel portion (semiconductorregion of the second conduction type), including:

(b1) a second conductor; and

(b2) a third conductor arranged between the second conductor and thesemiconductor substrate,

wherein the contact resistance between the source portion and theconductive portion is higher than the contact resistance between channelportion and the conductive portion.

The third conductor described above is formed of molybdenum silicide,for example, in a case where the first conduction type is an n-type.

Further, in a case where the first conduction type of the semiconductordevice is an n-type, a barrier height between the n-semiconductor andthe third conductor may be higher than a barrier height between then-semiconductor and titanium tungsten.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view for a main portion of a substrateshowing a method of manufacturing a semiconductor device of Embodiment 1according to the invention;

FIG. 2 is a plan view for a main portion of a substrate showing a methodof manufacturing a semiconductor device of Embodiment 1 according to theinvention;

FIG. 3 is a cross sectional view for a main portion of a substrateshowing a method of manufacturing a semiconductor device of Embodiment 1according to the invention;

FIG. 4 is a plan view for a main portion of a substrate showing a methodof manufacturing a semiconductor device of Embodiment 1 according to theinvention;

FIG. 5 a cross sectional view for a main portion of a substrate showinga method of manufacturing a semiconductor device of Embodiment 1according to the invention;

FIG. 6 a cross sectional view for a main portion of a substrate showinga method of manufacturing a semiconductor device of Embodiment 1according to the invention;

FIG. 7 is a plan view for a main portion of a substrate showing a methodof manufacturing a semiconductor device of Embodiment 1 according to theinvention;

FIG. 8 is a cross sectional view for a main portion of a substrateshowing a method of manufacturing a semiconductor device of Embodiment 1according to the invention;

FIG. 9 a cross sectional view for a main portion of a substrate showinga method of manufacturing a semiconductor device of Embodiment 1according to the invention;

FIG. 10 is a plan view for a main portion of a substrate showing amethod of manufacturing a semiconductor device of Embodiment 1 accordingto the invention;

FIG. 11 is a cross sectional view for a main portion of a substrateshowing a method of manufacturing a semiconductor device of Embodiment 1according to the invention;

FIG. 12 is a cross sectional view for a main portion of a substrateshowing a semiconductor device for comparison with the semiconductordevice according to Embodiment 1 of the invention;

FIG. 13 is a cross sectional view for a main portion of a substrateshowing a method of manufacturing a semiconductor device for comparisonwith the semiconductor device according Embodiment 1 of the invention;

FIG. 14 is a cross sectional view for a main portion of a substrateshowing a method of manufacturing a semiconductor device for comparisonwith the semiconductor device according Embodiment 1 of the invention;

FIG. 15 is a cross sectional view for a main portion of a substrateshowing a method of manufacturing a semiconductor device for comparisonwith the semiconductor device according Embodiment 1 of the invention;

FIG. 16 is a cross sectional view for a main portion of a substrateshowing the semiconductor device according to Embodiment 1 of theinvention:

FIG. 17 is a cross sectional view for a main portion of a substrateshowing a method of manufacturing the semiconductor device according toEmbodiment 1 of the invention;

FIG. 18 is a cross sectional view for a main portion of a substrateshowing a semiconductor device for comparison with the semiconductordevice according Embodiment 1 of the invention;

FIG. 19 is a table showing a relation between the value for barrierheight ΦB to n-silicon and a contact (ohmic) resistance;

FIG. 20 is a cross sectional view for a main portion of a substrateshowing a relation between an n-channel type power MISFET and an npntype parasitic bipolar transistor;

FIG. 21 is a schematic cross sectional view for a main portion of asubstrate showing a relation between a parasitic bipolar transistor andresistance Re, etc.;

FIG. 22 is a schematic cross sectional view for a main portion of asubstrate showing a relation between a parasitic bipolar transistor andresistance Re, etc.;

FIG. 23 is a graph showing a relation between IDSS [A] and VDSS [V] in acase of using TiW and in a case of using MoSi₂ as the material for abarrier film;

FIG. 24 is a cross sectional view for a main portion of a substrateshowing a semiconductor device according to Embodiment 1 of theinvention:

FIG. 25 is an equivalent circuit diagram in a avalanche state of ann-channel type power MISFET;

FIG. 26 is a circuit diagram showing an avalanche resistance amountmeasuring circuit of an n-channel type power MISFET;

FIG. 27 is a waveform chart showing a relation for VDS, IDS and VGS ofan n-channel type power MISFET;

FIG. 28 is a graph showing a relation between IAP [A] and the N [number]in a case where the material for the barrier film is TiW;

FIG. 29 is a graph showing a relation between IAP [A] and the N [number]in a case where the material for the barrier film is MoSi₂;

FIG. 30 is a table showing barrier height ΦB for various kinds of metalsto n-Si, which is for metals of higher barrier height ΦB than TiW;

FIG. 31 is a table showing barrier height ΦB for various kinds of metalsto n-Si, which is for metals of lower barrier height ΦB than TiW;

FIG. 32 is a circuit diagram showing a DC/DC converter using ann-channel type power MISFET,

FIG. 33 is a circuit diagram showing a DC/DC converter, using ann-channel type power MISFET, and a p-channel type power MISFET;

FIG. 34 is a perspective view and an internal circuit diagram of apackage in a case of forming an n-channel type power MISFET and ap-channel type power MISFET into one package;

FIG. 35 is a plan view for a main portion of a device in a case offorming an n-channel type power MISFET and a p-channel type power MISFETinto one package;

FIG. 36 is a cross sectional view for a main portion of a substrateshowing a method of manufacturing a semiconductor device according toEmbodiment 4 of the invention;

FIG. 37 is a cross sectional view for a main portion of a substrateshowing a method of manufacturing a semiconductor device according toEmbodiment 4 of the invention; and

FIG. 38 is a cross sectional view for a main portion of a substrateshowing a method of manufacturing a semiconductor device according toEmbodiment 4 of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention are to be described withreference to the drawings. Throughout the drawings for explaining thepreferred embodiments, those having identical functions carry identicalreference numerals for which duplicate description will be omitted.

Embodiment 1

A semiconductor device according to this embodiment is to be describedin accordance with a manufacturing method thereof.

FIG. 1 to FIG. 11 are cross sectional views for a main portion or planviews for a main portion of a substrate showing a method ofmanufacturing a semiconductor device according to this embodiment.

At first, as shown in FIG. 1, a semiconductor substrate 1 in which asingle crystal silicon layer 1 b doped with an n-impurity (for example,phosphorus) is epitaxially grown on the surface of an n-single crystalsilicon substrate 1 a (hereinafter simply referred to as “substrate”) isprovided.

Then, the substrate 1 is etched to form trenches 7 by using a filmpatterned by using photolithography (not illustrated) as a mask. FIG. 2shows an example of a pattern for the trenches 7. As shown in thedrawing, trenches 7 are arranged so as to surround the periphery ofplural octagonal patterns. The width of the trench 7 is, for example,about 0.5 μm. FIG. 1 corresponds, for example, to A-A cross section inFIG. 1.

Then, as shown in FIG. 3, a heat treatment is applied to the substrate 1to form a thermal oxide film 9 to the bottom and the side of thetrenches 7. The thermal oxide film 9 forms a gate dielectric film for apower MISFET. Then, a polycrystal silicon film 11 of low resistancedoped with an impurity is deposited to an extent of burying the trenches7. In this process, the polycrystal silicon film 11 is deposited in alayerous form at the periphery of a device forming region not formedwith the trench.

Then, the polycrystal silicon film 11 is etched by using a notillustrated photoresist film (hereinafter simply referred to as “resistfilm”) as a mask to leave the polycrystal silicon film 11 in thetrenches 7. The polycrystal silicon film 11 in the trench forms a gateportion G of a power MISFET. In this process, as shown in FIG. 4, apolycrystal silicon film pattern P is formed at the periphery of thedevice forming region. The pattern P is connected with the gate portionG at the periphery of the device forming region. CA denotes a chipregion.

Then, as shown in FIG. 5, using a not illustrated resist film as a mask,a p-impurity is implanted and diffused into the substrate 1 between thetrenches 7 to form a p⁻-semiconductor region (channel region, well) 15.The bottom of the trench 7 is at a position deeper than thep⁻-semiconductor region 15.

Then, with a not illustrated resist film as a mask, an n-impurity (forexample, As) is implanted into the substrate 1 between the trenches 7 toform an n⁺-semiconductor region (source region) 17. The n⁺-semiconductorregion (source region) 17 is formed in the plural octagonal patternregions shown in FIG. 2. Further, the bottom of the n⁺-semiconductor 17is in the p⁻-semiconductor region 15.

Then, as shown in FIG. 6 and FIG. 7, after forming a silicon oxide film19 above the substrate 1, the substrate 1 (p⁻-semiconductor region 15and the n⁺-semiconductor region 17) in the plural octagonal patterns (onn⁺-semiconductor region 17) is etched by using a not illustrated resistfilm as a mask to form contact trenches (source contact) 21 s. In thiscase, etching condition is controlled such that the diameter for theopening of the substrate 1 (17, 15) is smaller than the diameter for theopening of the silicon oxide film 19.

The n⁺-semiconductor region 17 is exposed from the lateral wall of thecontact trench 21 s and the p⁻-semiconductor region 15 is exposed fromthe bottom thereof. In other words, the depth of the contact trench 21 sexceeds the n⁺-semiconductor region 17 and reaches as far as thep⁻-semiconductor region 15.

Further, although not illustrated, the silicon oxide film 19 above thepolycrystal silicon film pattern P is removed to form a contact trench(gate contact).

Then, as shown in FIG. 8, a p-impurity, for example, boron fluoride(BF₂) is implanted to the bottom of the contact trench 21 s and diffusedto form a p⁺-semiconductor region (back gate contact region) 23. Thatis, the source electrode formed on the contact trench 21 s is connectedwith the source region 17, and connected with the back gate by way ofthe p⁺-semiconductor region 23.

By forming the contact trench 21 s and disposing the p⁺-semiconductorregion 23 at the bottom thereof as described above, mask alignmentmargin can be decreased to refine the portion between the gates.

For example, as shown in FIG. 12, it is also possible to form ap⁺-semiconductor region 23 on the surface of the substrate.

In this case, however, as shown in FIG. 13, after at first forming ap⁻-semiconductor region (channel region) 15, a resist film R1 is formedon a portion between the gate portions G (plural hexagonal patterns) soas to cover the region for forming the p⁻-semiconductor region 15. Then,an n-impurity (for example, arsenic) is implanted and diffused to forman n⁻-semiconductor region 17 by using the resist film R1 as a mask.

Further, as shown in FIG. 14, after forming a silicon oxide film 19above the substrate 1, contact trenches (source contact) 21 s are formedby using a resist film R2 having an opening above a portion between then⁺-semiconductor regions 17 as a mask and, further, a p-impurity, forexample, boron fluoride (BF₂) is implanted and diffused into theopenings to form p⁺-semiconductor regions 23.

As described above, in the structure shown in FIG. 12, mask alignment isapplied twice, so that mask alignment or the like has to be taken intoconsideration and refinement of the device is difficult. FIG. 15 showsan example for distance between each of the portions. In the drawing,are shown a barrier film 25, a source electrode SE comprising analuminum film 27, and a conductive film 35 forming a drain electrode.

Portion A indicates a distance between the end of the contact trench 21s and the end of the n⁺-semiconductor region 17, and it requires adistance of about 0.6 μm in view of the alignment accuracy between eachof the three layers, i.e., the gate portion G, the n⁺-semiconductorregion 17 and the contact trench 21 s.

Further, as the portion between the gate portions is refined, the widthof the resist film R1 for instance (refer to FIG. 13) is decreased and,in a case where the width is 1.0 μm or less, peeling of the resist filmoccurs making it impossible to implant the impurity to desired regions.Accordingly, about 1.0 μm is necessary for the portion B (width of thep⁺-semiconductor region 23).

Further, portion C is a distance between the end of the gate portion Cand the end of the contact trench 21 s, and the distance of 0.45 μm ormore is necessary in order to prevent short circuit between the gateportion G and the source electrode SE in the contact trench 21 s.

Accordingly, the cell size in this case is 4.2 μm. The cell size CS isdefined as a center-to-center distance for the adjacent gate portions G,that is, the sum for the width of the trench 7 (1.1 μm in FIG. 15) andthe distance of portion A, portion B and portion C.

Accordingly, in a case of decreasing the cell size, for example, to 3 μmor less, it is necessary to adopt a structure shown in FIG. 16.

In this case, it is not necessary to form the resist film R1 to aportion between the gate portions G in order to cover the region forforming the p⁻-semiconductor region 15 as shown in FIG. 17. That is, theimpurity can be implanted for the entire cell region. That is, only themask alignment error upon forming the contact trench 21 has to be takeninto consideration for the mask alignment error. Accordingly, as shownin FIG. 16, the portion A can be decreased to about 0.1 μm, and theportion B can also be decreased within such a range as enablingconnection with the substrate. A distance of 0.45 μm or more isnecessary for the portion C in order to prevent short circuit betweenthe gate portion G and the source electrode SE.

In this case, also in a case of ensuring the portion B by about 1.0 μmin order to decrease the connection resistance, the cell size can berefined as about 3.2 μm.

Then, as shown in FIG. 9, a molybdenum silicide (MoSi₂) film 25 isdeposited as a barrier film by about 60 nm on the silicon oxide film 19including the inside of the contact trench 21 s, for example, bysputtering.

Successively, an aluminum (Al) film 27, for example, is deposited as aconductive film to about 5 μm by sputtering. The barrier film is formedfor preventing silicon in the substrate 1 from depositing into the Alfilm 27. The Al film may comprise Al as a main ingredient and it mayalso contain other metals.

Then, the MoSi₂ film 25 and the Al film 27 are etched by using a notillustrated resist film as a mask to form a gate electrode (gate leadelectrode) GE and a source electrode (source lead electrode) SE. Theelectrodes (GE, SE) form first layer wirings. FIG. 10 is an example forthe pattern of the gate electrode GE and the source electrode SE.

Then, as shown in FIG. 11, a protection film, for example, a polyimidefilm 29 is coated above the substrate 1, and the polyimide film on thegate electrode GE and the source electrode SE are removed by exposureand development to form an opening (pad portion). FIG. 11 shows anopening 31 s on the source electrode SE.

Then, after protecting the surface of the substrate 1 by a tape or thelike, the rear face of the substrate 1 is ground with the protectionsurface being on the lower side. Then, for example, an Ni (nickel) film,a Ti (titanium) film and a gold (Au) film are formed as conductive filmsuccessively on the rear face (la) of the substrate 1 by sputtering toform a stacked film 35 thereof. The stacked film 35 forms a leadelectrode (drain electrode DE) for the drain (1 a, 1 b).

Then, the tape is peeled and the substrate 1 in the state of a wafer issubject to dicing, for example, along the chip region CA, individualchips are mounted on the side of the electrode DE thereof on a leadframe (mounting plate), for example, having an external terminals, andthe external terminals and the gate electrode GE and the sourceelectrode SE exposed from the pad portion are connected by utilizinggold wires or bumps. Then, the periphery of the chip is sealed by aresin or the like.

As a result, a semiconductor device having an n-channel type powerMISFET is completed. The completed figure is not illustrated.

As described above according to this embodiment, since the barrier filmof the n-channel type power MISFET is constituted with MoSi₂, IDSS (leakcurrent between drain and source when a voltage is applied to the drainin a state where voltage between gate and source is 0 V) can bedecreased. Further the avalanche resistance amount can be improved.

The reason capable of obtaining the effects described above is to beexplained below.

(1) The effect of decreasing IDSS (leak current) is to be described.Description is to be made at first to a case studied on the barriermaterial before adopting MoSi₂ made by the present inventors.

The present inventors studied for the adoption of titanium tungsten(TiW) film as the barrier material. However, in a case of using a TiWfilm 325 as a barrier film as shown in FIG. 18 for example, IDSS wasincreased by about one digit compared with the device of the structureshown in FIG. 12. The thickness of the TiW film in this case was about150 to 200 nm.

As described above, the channel width per unit area is increased by therefinement of the cell. Accordingly, while increase of IDSS was expectednaturally, since IDSS increased remarkably exceeding the increment ofthe channel width, constitution and the material for each of theportions were studied. As a result, IDSS could be decreased successfullyby using MoSi₂ as described previously.

This is considered to be attributable to that, in a case of using MoSi₂as the barrier film, MoAl₁₂ as a compound (reaction product) of Al as anupper layer and Mo (molybdenum) is formed at the boundary with thesubstrate 1 (Si: silicon).

The barrier height ΦB for MoAl₂ and n-Si is about 0.65 V. In a casewhere a metal and a semiconductor are in contact with each other, thebarrier height ΦB means a potential barrier at the boundarytherebetween.

On the other hand, in a case of using TiW with a higher W (tungsten)constitutional ratio as a barrier film, it is considered that thebarrier height ΦB is equivalent with barrier height ΦB for W and n-Siwhich is about 0.70 V.

FIG. 19 shows a relation between the value for the barrier height ΦB andthe contact resistance. As illustrated, contact resistance to n-Si ishigher in a case of using MoSi₂ compared with a case of using TiW and,on the other hand, the contact resistance to n-Si is lower in a case ofusing TiW compared with a case of using MoSi₂.

The relation described above is reversed with respect to p-Si, i.e., thecontact resistance to p-Si is lower in a case of using MoSi₂, while thecontact resistance to p-Si is higher in a case of using TiW.

In an n-channel type power MISFET, as shown in FIG. 20, an npn-typeparasitic bipolar transistor PB constituted with an n⁺-semiconductorregion (source region) 17, a p⁻-semiconductor region (channel region)and a drain (1 a, 1 b) is present inside. G, S and D mean gate, sourceand drain, respectively.

BVDSS of the n-channel type power MISFET (breakdown voltage fordrain-channel junction when voltage is applied to the drain in a statewhere voltage between gate and source is 0 V) is equivalent withBV_(CES) of the bipolar transistor (breakdown voltage for collector-basejunction when voltage is applied to the collector in a state wherevoltage between base and emitter is 0 V), and is substantiallyequivalent with BV_(CBO) (junction withstand voltage forp⁻-semiconductor region 15 and the drain 1 b) in a structure where theMISFET does not punch through.

The bases of the parasitic bipolar transistor PB (p⁻-semiconductorregion 15 and p⁺-semiconductor region 23) are connected electricallywith the Al film 27 by way of the barrier film 25. That is, since theemitter (n⁺-semiconductor region 17) and the base (p⁻-semiconductorregion 15) are short circuited by the source electrode SE in thisstructure, it is ideal that the regions are theoretically at anidentical potential.

However, in a case of using a metal such as TiW having a higher contactresistance to p-Si, that is, a metal having lower ΦB to n-Si such as TiWas the barrier film, a high resistance Re is added to the bases(p⁻-semiconductor region 15 and p⁺-semiconductor region 23) as shown inFIG. 21.

As a result, the emitter Em and the base Ba are more tended to be biasedforwardly. That is, the parasitic bipolar transistor tends to take anon-state to increase the leak current (IDSS).

On the other hand, in a case of using, as the barrier film, a metal suchas MoSO₂ having a higher contact resistance to n-Si, that is, a metalhaving higher ΦB to n-Si, a higher resistance Re is added to the emitter(n⁺-semiconductor region 17) as shown in FIG. 22.

As a result, the emitter Em and the base Ba are more tended to be biasedbackwardly. That is, the parasitic bipolar transistor tends to take anoff-state to decreases the leak current (IDSS).

FIG. 23 shows a result of examining a relation between IDSS[A] andVDSS[V] by using TiW and MoSi₂ as the material for the barrier film eachof an identical chip size and each by the number of ten samples. Asshown in the graph, in a case of using MoSi₂ as the barrier film (graph(a)) IDSS can be lowered than in a case of using TiW for the barrierfilm (graph (b)). For example, in a case of using TiW for the barrierfilm at VDSS of 30.5 V and VGS (gate potential) of 0 V, IDSS was 2.15 nAto 3.74 nA, whereas in a case of using MoSi₂ for the barrier film, IDSSwas from 742 pA to 918 pA. As described above, in a case of using MoSi₂leak current (IDSS) could be decreased to about ⅓ of that in a case ofusing TiW.

As described above, according to this embodiment, since a metal ofhigher barrier height ΦB than TiW to n-Si is used for the barrier filmof the n-channel type power MISFET, the leak current (IDSS) can bedecreased.

Further, since the metal having lower contact resistance to p-Si (23 or15) than the contact resistance to n-Si (17) is used, the leak current(IDSS) can be decreased.

While description has been made to an example of the n-channel typepower MISFET, the leak current can be decreased also in a p-channel typepower MISFET by properly selecting the material for the barrier film.FIG. 24 shows a cross sectional view for a main portion of a p-channeltype power MISFET. In this case, while the conduction type of theimpurity for each of the portions constituting the p-channel type powerMISFET is different, it can be formed by the identical process for then-channel type power MISFET. 201 denotes a substrate comprising ap-single crystal silicon substrate 201 a and a p-single crystal siliconlayer 201 b. Further, there are also shown an n-semiconductor region(channel region, well) 215, a p⁺-semiconductor region (source region)217, and an n⁺-semiconductor region (back gate contact region) 223.

That is, in a case of using a metal having lower contact resistance ton-Si, that is, having lower ΦB to n-Si for the barrier film 225, a highresistance Re is added to the emitter. As a result, the emitter and thebase are more tended to be biased backwardly. That is, the parasiticbipolar transistor tends to take an off-state to decrease the leakcurrent (IDSS).

As described above, in the p-channel type power MISFET, the leak current(IDSS) can be decreased by using a metal of lower barrier height ΦB ton-Si (for example, 0.65 or less in a case of TiW) as the barrier film.

Further, the leak current IDSS can be decreased by using a metal ofhigher contact resistance to p-Si (217) than the contact resistance ton-Si (223 or 215).

(2) Then, the effect of improving the avalanche resistance amount is tobe described below.

FIG. 25 shows an equivalent diagram for the avalanche state, FIG. 26shows a circuit for measuring avalanche resistance amount and FIG. 27shows waveforms for VDS (drain-source voltage: solid line), IDS(drain-source current: dotted chain), and VGS (gate-source voltage:broken line).

FIG. 26 shows an example of a circuit for measuring avalanche resistanceamount in which a dielectric load L such as a transformer (hereinafterreferred to as L load) is connected to the drain of a power MISFET Qngrounded at the source, and the L load is connected with a power sourceVDD grounded on one side. As shown in FIG. 27, when a power sourcevoltage VDD is applied to a power MISFET and when the power MISFET isturned on (gate ON), VDS takes an extremely low value (on voltage VDS(on) value) and IDS increases gradually. Then, when the power MISFET isturned off (gate OFF), a counter electromotive force is generated to theL load in which VDS increases abruptly by the counter electromotiveforce to reach the breakdown voltage of drain-channel junction (VDSS)and then keeps BVDSS for a period till the energy accumulated in the Lload is consumed (avalanche period ta). In this case, as shown in FIG.25, a great amount of carriers (electrons and holes) are generated atthe boundary of the pn junction (junction between p⁻-semiconductorregion 15 and the substrate 1 b), to cause an avalanche current to flowin which electrons pass toward the substrate (1 a, 1 b, collector Co,drain) while holes pass toward the p⁻-semiconductor region 15 (base Ba,channel) That is, the energy accumulated the L load is consumed by thebreakdown of the pn junction in the power MISFET for the avalancheperiod ta. Accordingly, in the avalanche period ta, IDS decreasesgradually and VDS decrease to the power source voltage VDD at theinstance IDS decreases to zero. In this case, the energy that can beconsumed by the semiconductor region in the power MISFET is referred toas an avalanche resistance amount. As has been described in (1) abovewith reference to FIG. 20 and FIG. 21, when the contact resistance ishigh to p-Si, the base resistance (diffusion resistance of thep⁻-semiconductor region 15 with respect to the Al film 27) is increasedto cause voltage drop. As a result, the parasitic bipolar transistordescribed specifically (1) in above is operated. As has been describedpreviously, a number of MISFET cells are connected in series in thepower MISFET, in which a cell tending to turn-ON depending on thevariation of characteristics of each of parasitic npn transistors ispresent. That is, current is concentrated to the cell of higher baseresistance to destroy the power MISFET. This is referred to as avalanchebreakdown.

Accordingly, for improving the avalanche resistance amount, it isnecessary to make the parasitic bipolar transistor less tending toturn-ON.

Then, for reducing the base resistance, a metal is used which has higherbarrier height ΦB than TiW with respect to n-Si (for example, MoSi₂) forthe barrier film of the n-channel type power MISFET, so that theavalanche resistance amount can be improved.

Further, the avalanche resistance can be improved by using a metalhaving a lower contact resistance to p-Si than contact resistance ton-Si.

On the other hand, in the p-channel type power MISFET, the avalancheresistance amount can be improved by using a metal having a lowerbarrier height to n-Si as the barrier film (for example, barrier heightΦB of 0.65 or less).

Further, the avalanche resistance amount can be improved by using ametal having a higher contact resistance to p-Si than contact resistanceto n-Si.

An example of a method of measuring the avalanche resistance amountincludes a method of measuring an avalanche breakdown current. Theavalanche breakdown current is an IDS value just before the breakdown ofa device in a case where the on period for the power MISFET is graduallyextended by a pulse generator P.G. and the energy to the power MISFET iskept to be increased gradually till the device is broken down whilekeeping the L load constant in FIG. 26. Rg and Rgs represent resistanceeach of which is about 50Ω. Further, VDS and IAP represent measuringterminals.

FIG. 28 is a graph showing a relation between the avalanche breakdowncurrent [A] and the number N [number] in a case of using TiW as thematerial for the barrier film of the n-channel type power MISFET. FIG.29 is a graph in a case of using MoSi₂ as the material for the barrierfilm. VDD is 15 V, VGS is 15 V, L is 100 μH and N is 40.

AS shown in FIG. 28 and FIG. 29. In a case of using MoSi₂ for thebarrier material, the avalanche resistance amount (avalanche break downcurrent) can be improved. In a case of FIG. 28, the maximum value (Max)is 5 A, the minimum value (Min) is 1 A and the average value thereof is2.6 A for the avalanche breakdown current. On the contrary, in a case ofFIG. 29, the maximum value is 65 A, the minimum value is 29 A and theaverage value is 35.6 A for the avalanche breakdown current.

The power MISFET is used as a high speed switching device, for example,to a switching power source circuit or a DC/DC converter. In the powerMISFET described above, as the operation frequency goes higher, a surgevoltage (over voltage exceeding drain withstanding voltage) of a narrowpulse width tends to be generated upon turn-OFF by the inductance in thecircuit and stray inductance for wirings.

For absorbing the surge voltage, it is suitable to use this embodimentsince high avalanche resistance amount is necessary for the powerMISFET.

(3) As has been described above specifically, in the n-channel typepower MISFET, the leak current can be decreased and the avalancheresistance amount can be improved by using MoSi₂ as the barrier filmcompared with the case of using TiW.

However, use of MoSi₂ may possible cause a worry of electro-migrationresistance and may possibly cause a problem of deterioration in thecoverage. The problems described above are overcome in this embodimentby the countermeasures to be described below.

Generally, it is considered that TiW has electro-migration (EM)resistance and can improve the wiring life. However, in the powerMISFET, the pattern for the gate electrode GE and the source electrodeSE is large and the current density is not high as has been describedwith reference to FIG. 10. Also in the contact portion, since cells ofseveral tens of thousands are connected in parallel, current flowing perunit contact is not large compared with usual integrated circuits.

Accordingly, also in a case of using MoSi₂, EM resistance can bemaintained.

Further, the thickness can be increased in the TiW film and it can beformed, for example, to about 150-200 nm. On the contrary, in a case ofusing MoSi₂, increase in the film thickness is difficult since it isconsidered that Si constituting MoSi₂ invades into the Al film toincrease the resistance of Al wirings. Accordingly, the barrier propertyis deteriorated to possibly lower the coverage for the contact trench 21s.

In view of the above, in this embodiment, the diameter for the contacttrench 21 s is made to about 1.0 μm and the film thickness of MoSi₂ wasmade to 60 nm.

According to the study of the present inventors, even when the thicknessof the MoSi₂ film was about 60 nm, there was found no increase in theresistance in the Al wirings that would give rise to the problem in viewof operation.

As described above, the barrier property and the coverage of MoSi₂ canbe improved by increasing the thickness of the MoSi₂ film to some extentand decreasing the aspect ratio of the contact trench.

Naturally, even when the aspect ratio is large, coverage can beimproved, for example, by the improvement for the film deposition methodor improvement for the performance of the film deposition apparatus.

Embodiment 2

Description has been made for Embodiment 1 to an example of using MoSi₂as a metal having higher barrier height ΦB than TiW to n-Si used for thebarrier film of the n-channel type power MISFET. In this embodiment,barrier height ΦB between various kinds of metals and n-Si are examined,and description is to be made for barrier films which are used suitablyto the n-channel type power MISFET or p-channel type power MISFET.

FIG. 30 is a table showing the barrier height ΦB [V] of various kinds ofmetals to n-Si, which is a table for metals having higher barrier heightΦB than that in the case of using TiW as the barrier film (barrier metalfilm).

In a case of using metals described in the table, i.e., Co (cobalt), Ni(nickel), Rh (rhodium), MoAl_(x), Pb (lead), Mn (manganese), Pt(platinum) or Ir (iridium) as the barrier film, the barrier height ΦB ton-Si is high and the leak current can be decreased and the avalancheresistance amount can be improved, for example, by using the barrierfilm 25 shown in FIG. 11. The metals are not necessarily elementalmetals but an impurity may be contained in the film or metals may bechemically bonded with other elements so long as the metals describedabove are main constituent metal in the film. Further, in a case wherethe metals can react with elements constituting the substrate or theupper electrode (source electrode), a consideration is taken for thebarrier height ΦB between the reaction product and the substrate.

Among them, since Co and Ni are metals often used in the field ofsemiconductors and various studies have been made for thecharacteristics thereof, they are suitable to be used as the barrierfilm. Further, various film deposition methods have also been studiedand, for example, they can be easily deposited into films by using asputtering apparatus. Further, they are inexpensive and can reduce themanufacturing cost of the apparatus. For Co and Ni, there is a certainrange for the numerical values of the barrier height ΦB. This is becausethe numerical values vary depending on the conditions for forming themetals and silicide layers. Accordingly, it is necessary to control theforming conditions such that the barrier height ΦB becomes higher (forexample, so as to be 0.65 or higher).

Further, since Pt is also used often in the field of semiconductors andvarious studies have also been made for the characteristics and the filmdeposition method therefor, so that it is suitably used for the barrierfilm. It has an outstandingly high barrier height ΦB and is usedsuitably for the barrier film.

Further, Pb has a lower melting point and can not be used for apparatusrequiring high temperature heat treatment step after formation of thebarrier film. Further, in the use of Mn or Rh, their characteristics orfilm deposition methods have to be taken into consideration. Further,while Ir has high barrier height ΦB value and is prospective,characteristics and film deposition methods therefor have to be takeninto consideration.

FIG. 31 is a table showing the barrier height ΦB of various kind ofmetals to n-Si, which is a table for metals having lower barrier heightΦB than that in the case of using TiW for the barrier film.

In a case of using metals described in the table, that is, Co (cobalt),Ti (titanium), Ta (tantalum), Cr (chromium), Mo (molybdenum), Zr(zirconium), and Hf (hafnium) as the barrier film, the barrier height ΦBto n-Si is low and the leak current can be decreased and the avalancheresistance amount can be improved when used for the barrier film 225 ofthe p-channel type power MISFET shown in FIG. 24.

The metals are not necessarily elemental metals but an impurity may becontained in the film or metals may be chemically bonded with otherelements so long as the metals described above are main constituentmetal in the film. Further, in a case where the metals can react withelements constituting the substrate or the upper electrode (sourceelectrode), a consideration is taken for the barrier height ΦB betweenthe reaction product and the substrate.

Among them, Since Ti is a metal often used in the field ofsemiconductors and various studies have been made for thecharacteristics thereof, it is used suitably as the barrier film.Further, various film deposition methods have also been studied andfilms can be deposited easily, for example, by using a sputteringapparatus. Further, it is inexpensive to reduce the production cost ofthe apparatus. In addition to Ti, TiN (titanium nitride) may also beused.

Further, various studies have also been made for the characteristics andthe film deposition methods of Co, which is inexpensive as well, Co issuitably used for the barrier film. As has been described above, thereis a certain range for the numerical value of barrier height ΦB for Co.Accordingly, it is necessary to control the forming conditions such thatthe barrier ΦB is lower (for example, so as to be 0.65 or less).

Further, since Cr and Mo are also used often in the field ofsemiconductors and various studies have been made for thecharacteristics and the film deposition methods, they are suitably usedfor the barrier film. Further, they have relatively lower barrier heightΦB and used suitably for the barrier film. However, in a case of usingMo for the barrier film and an Al film for the upper electrode, sinceMoAl_(x) is formed to improve the barrier height ΦB, it is necessary totake a consideration, for example, use of a metal film other than Alfilm or formation of a reaction preventive film relative to the Al film.Further, since Ta is also used often in the field of semiconductors andvarious studies have been made for the characteristics and the filmdeposition method, it is suitably used for the barrier film. TaN(tantalum nitride) may also be used. Further, Zr and Hf have lowerbarrier height ΦB values and are prospective but consideration have tobe taken on the characteristics and film deposition method.

In this embodiment, description has been made to an example of thebarrier height ΦB of various kinds of metals to n-Si, but relationbetween ΦBn to n-semiconductor and ΦBp to p-semiconductor can berepresented by the equation:ΦBp=Eg/q−ΦBnin which Eg represents a band gap and q represents a charge amount ofelectrons.

For example, since Si band gap is 1.12 eV, ΦBp to p-Si can be expressedby the equation:ΦBp=1.12−ΦBn

Accordingly, in a case, for example, of Ti, since ΦB to n-Si is 0.60 V,ΦB relative to p-Si is 0.52 V. Further, for example, in a case of Cr,since ΦB relative to n-Si is 0.57 V, ΦB relative to p-Si is 0.55 V.

As described above, the sequence for the magnitude of the barrier heightΦB of metals relative to n-Si is reversed with respect to the case ofp-Si.

Embodiment 3

For example, as shown in FIG. 32, the DC/DC converter includes astructure in which an n-channel type power MISFET Q1 and Q2 areconnected in series between an input potential Vin and the groundpotential GND. L represents an inductance which is located between theconnection node of Q1 and Q2 and the output potential Vout. Further, acapacitance C1 is connected between the ground potential GND and thepower potential Vout. Further, a built-in diode is connected between thesource and the drain of the n-channel type power MISFET Q1 and Q2.

The side of the input potential Vin is referred to as a high side whilethe side of the ground potential GND is referred to as a low side.

The n-channel type power MISFET Q1 and Q2 are driven by a controller ICconnected between the power source potential Vcc and the groundpotential GND.

For example, the n-channel type power MISFET Q1 on the high side isdriven by a charge pump circuit CP that outputs a voltage higher thanthe source potential. The charge pump circuit CP is generally build-inthe controller IC.

However, there is a DC/DC converter using the p-channel type powerMISFET on the high side in order to save the charge pump circuit CP.

FIG. 33 shows a circuit diagram for a DC/DC converter using thep-channel type power MISFET Q3 on the high side. Constitutions otherthan the p-channel type power MISFET Q3 and the charge pump circuit CPare identical with those in FIG. 32.

The barrier film of the p-channel type power MISFET Q3 and the n-channeltype power MISFET Q2 in the circuit described above can be optimized byconsidering the barrier height ΦB.

For example, by using a metal having a relatively higher barrier heightΦB for the barrier film of the n-channel type power MISFET Q2 (forexample, MoSi₂) and using a metal having relatively lower barrier heightΦB to the n-Si for the barrier film of the p-channel type power MISFETQ3 (for example, TiN), the leak current of the power MISFET (Q3, Q2) canbe decreased, and the avalanche resistance amount can be improved toimprove the characteristics of the DC/DC converter.

Further, as shown in FIG. 34, the power MISFET (Q3, Q2) may be formedinto one package. The upper portion of the drawing is a perspective viewfor a package in which the n-channel type power MISFET and the p-channeltype power MISFET are formed into one package and a lower portionthereof is an internal circuit diagram.

In the drawings, T1-T8 represent external terminals in which T1 isconnected with the source electrode SE of Q2, T2 is connected with thegate electrode GE of Q2, and T7 and T8 are connected with the drainelectrode DE of Q2. Further, T3 is connected with the source electrodeSE of Q3, T4 is connected with the gate electrode GE of Q3, T5 and T6are connected with the drain electrode DE of Q3. FIG. 35 shows anexample of a plan view for a main portion of the apparatus shown in FIG.34. The drain electrodes DE is formed on the rear face of each of chipsand connected by way of a die pad to an external terminal (T8 and T7,and T6 and T5).

That is, for example, the n-channel type power MISFET Q2 is constitutedas shown in FIG. 11, while the p-channel type power MISFET Q3 isconstituted as shown in FIG. 24, and they are die bonded on die pads ofa lead frame, connected by using external terminals and gold wires andthen the periphery of the chip is resin-sealed.

As described above, in the device incorporating the n-channel type powerMISFET and the p-channel type power MISFET, characteristics of thedevice can be improved by using different films for the respectivebarrier films, while using a metal having a relatively higher barrierheight ΦB to n-Si (for example, MoSi₂) in the case of the n-channel typeand using a metal having a relatively lower barrier height ΦB to then-Si (for example, TiN) in the case of the p-channel type.

Embodiment 4

In Embodiment 1, coverage of MoSi₂ is improved by ensuring a largediameter for the contact trench 21 s and decreasing the aspect ratio,but the coverage can be improved by taking a consideration for the shapeof the contact trench as shown below.

In FIG. 36, the width and the height of the silicon oxide film 19remaining on the gate portion G are decreased with the upper portionthereof being rounded.

For example, after forming the silicon oxide film 19 as shown in FIG. 6,a silicon oxide film 19 and the substrate 1 (p⁻-semiconductor region 15and n⁺-semiconductor region 17) are etched to form contact trenches(source contact) 21 s and then over etching is conducted under thecondition where the selectivity of the silicon oxide film 19 to thesubstrate 1 is high (silicon oxide film 19 is etched more easily).

As a result, the aspect ratio of the contact groove 21 s is decreasedand the step above the gate portion G is moderated to improve thecoverage of the barrier film 25, for example, of MoSi₂.

Further, in FIG. 37, side walls of the contact trenches 21 in thesubstrate 1 are rounded.

For example, after forming a silicon oxide film 19 as shown in FIG. 6,an opening is formed in the silicon oxide film 19 and, further, thesubstrate is dry etched while forming a great amount of polymer on thelateral surface.

Further, in FIG. 38, the bottom of the contact trench 21 s is rounded.

For example, after forming a silicon oxide film 19 as shown in FIG. 6,the silicon oxide film 19 and the substrate 1 (p⁻-semiconductor region15 and n⁺-semiconductor region 17) are etched to form a contact trench(source-contact) 21 s and, further, the bottom of the contact hole 21 sis isotropically etched to round the bottom thereof.

As a result, coverage of the barrier film 25, for example, of MoSi₂ canbe improved.

In FIG. 36 to FIG. 38, since constitutions and manufacturing methodsother than the contact trenches 21 or the barrier film 25 are identicalwith those of Embodiment 1 (for example, referred to FIG. 9)explanations therefor are to be omitted.

By the consideration on the shape of the contact trench as describedabove, coverage can be improved.

The inventions made by the present inventors have been describedspecifically with reference to the preferred embodiments but theinvention is not restricted to the embodiments and it will be apparentthat the invention can be changed variously within a range not departingthe scope thereof.

For example, in Embodiment 1, while the pattern for the trenches 7 isdefined to the shape shown in FIG. 2, various modifications arepossible, for example, by making the octagonal pattern 22 into atetragonal shape, or making the pattern for the trenches into astripe-like shape.

Further, in Embodiment 1 described above, while the gate portion is madein a trench structure, a planar type may also be provided additionally.

For example, the gate portion G in FIG. 11 is formed by patterning apolycrystal silicon film formed on a semiconductor substrate by way of agate insulation film. In this case, n⁺-semiconductor region (sourceregion) 17 is formed in the semiconductor substrate on both sides of thegate portion. The source region may not necessary be formed on bothsides but on any one side of the gate portion. Since constitutions forother portions are identical with those in Embodiment 1 (for examplereferred to FIG. 1), explanations therefor will be omitted.

Further, it is applicable not only to the power MISFET but also toMISFET in which the channel region (well), and the source or the drainregion are electrically connected.

That is, a gate electrode is formed by patterning a polycrystal siliconfilm formed by way of a gate insulation film to the main surface of awell formed in a semiconductor substrate and, further, source-drainregions are formed in the semiconductor substrate on both sides of gateelectrode. For example, the embodiment described above may also beapplied to a conductive portion connected electrically with the sourceor drain and well (for example, plugs or wiring portions). Further, alsoin this case, the gate portion of MISFET may be constituted into atrench structure.

The effects obtained by typical inventions among those disclosed in thepresent application are briefly describe as below.

Since the conductive portion in contact with the source portion and thechannel portion of MISFET is constituted with the first conductor and asecond conductor arranged between the first conductor and thesemiconductor substrate (source portion and channel portion), in whichthe contact resistance between the source portion and the conductiveportion is made higher than the contact resistance between the channelportion and the conductive portion, the leak current can be decreasedand the avalanche resistance amount can be improved in the MISFET.

Further, the characteristics of the semiconductor device having MISFETcan be improved.

1-37. (canceled)
 38. A method of manufacturing a semiconductor devicehaving a trench gate type MISFET, comprising the steps of: (a) providinga semiconductor substrate with a semiconductor layer formed over a mainsurface of said semiconductor substrate; (b) forming a plurality of gatetrenches of the MISFET on a main surface of the semiconductor layer; (c)forming a plurality of gate dielectric films in the gate trenchesrespectively; (d) forming a plurality of gate portions on the gatedielectric films respectively; (e) forming a plurality of channelregions of the MISFET, in the semiconductor layer, between adjacent gateportions respectively; (f) forming a plurality of source regions of theMISFET, in the semiconductor layer, over the channel regionsrespectively; (g) forming an insulating film over the source regions andgate portions; (h) forming a plurality of contact trenches, in theinsulating film and semiconductor layer, to expose the source regionsand channel regions; (i) forming a first conductive film in the contacttrenches to contact with the source regions and channel regions; and (j)forming a source electrode on the barrier film, wherein thesemiconductor substrate, semiconductor layer and source regions have afirst conduction type; wherein the channel regions have a secondconduction type; wherein the source electrode is comprised of a secondconductive film; and wherein the first conductive film and the sourceelectrode are formed such that a parasitic bipolar transistor, whichprovides a leakage current path between one of the source regions and adrain region of the MISFET, takes an off-state under a predeterminedcondition which would otherwise cause said parasitic bipolar transistorto take an on-state.
 39. A method of manufacturing a semiconductordevice according to claim 38, wherein said predetermined condition is acondition in which a voltage is applied to said drain region and avoltage between said one source region and a respective gate region iszero.
 40. A method of manufacturing a semiconductor device according toclaim 38, wherein a material of the first conductive film is selected toprovide a barrier height, with respect to a material of thesemiconductor layer, which is effective to cause the parasitic bipolartransistor to take the off-state.
 41. A method of manufacturing asemiconductor device according to claim 40, wherein said barrier heightis provided by a reaction product of the material of the firstconductive film and a material of the second conductive film.
 42. Amethod of manufacturing a semiconductor device according to claim 38,wherein the parasitic bipolar transistor has an emitter regioncorresponding to said one source region of the MISFET, a base regioncorresponding to a respective channel region of the MISFET, and acollector region corresponding to said drain region of the MISFET.
 43. Amethod of manufacturing a semiconductor device according to claim 38,wherein the insulating film comprises a silicon oxide film.
 44. A methodof manufacturing a semiconductor device according to claim 38, whereinthe semiconductor layer comprises an epitaxial film.
 45. A method ofmanufacturing a semiconductor device according to claim 44, wherein theepitaxial film is a silicon film.
 46. A method of manufacturing asemiconductor device according to claim 45, wherein the epitaxialsilicon film is a single crystal silicon film.
 47. A method ofmanufacturing a semiconductor device according to claim 38, wherein thesecond conductive film comprises aluminum.
 48. A method formanufacturing a semiconductor device according to claim 40, wherein thefirst conduction type is n-type, the second conduction type is p-type,and said first conductive film is mainly comprised of a materialselected from the group consisting of Co, Ni, Rh, Mo, Pb, Mn, Pt, Ir,and combinations thereof.
 49. A method of manufacturing a semiconductordevice according to claim 40, wherein the first conduction type isp-type, the second conduction type is n-type, and said first conductivefilm is mainly comprised of a material selected from the groupconsisting of Co, Ti, Ta, Cr, Mo, Zr, Hf, and combinations thereof. 50.A method of manufacturing a semiconductor device according to claim 38,wherein a diameter of the contact trench formed in the insulating filmis larger than that of the contact trench formed in the semiconductorlayer.
 51. A method of manufacturing a semiconductor device according toclaim 38, wherein the first conductive film and the source electrode areformed by a sputtering method in the steps (i) and (j), respectively.52. A method of manufacturing a semiconductor device according to claim38, further comprising the step of: (k) forming a drain electrode on arear surface of the semiconductor substrate.
 53. A method ofmanufacturing a semiconductor device according to claim 38, furthercomprising the step of: (l) between the steps (h) and (i), forming aback gate contact region under a bottom of each of the contact trenchesso as to contact with the bottom of each of the contact trenches,wherein the back gate contact region has the second conduction type; animpurity concentration of the back gate contact region is higher thanthat of the respective channel region; and the source electrode iselectrically connected with the back gate contact region.
 54. A methodof manufacturing a semiconductor device according to claim 38, whereinthe plurality of gate trenches are integrally formed; the plurality ofgate portions are integrally formed; and the plurality of source regionsare electrically connected.